The invention is directed to a process for the hydrophobization of finely divided silica and silicas by treating of an acidic aqueous suspension of the oxide with an organohalosilane at temperatures in the range of 50.degree. to 90.degree. C., filtering of the solid portion, washing and drying of the filter cake and subsequently tempering at temperatures in the range of about 300.degree. to 400.degree. C.
It is known to hydrophobize finely divided wet precipitated silica by a so-called coating. For this purpose one can operate, for example, by allowing a silicone oil to act on either the aqueous suspension of a precipitated silica or on a suspension of the dry powder in an organic solvent. It is also known to emulsify silicone oils in amounts up to 10% in a sodium silicate solution and to precipitate from this a modified silicate with a heavy metal salt which forms a difficultly soluble compound with silica. It is further known to hydrophobize silica or silicates by vaporization of organohalosilanes. Besides it is known to treat a mixed solution of 90% sodium silicate and 10% sodium methyl siliconate with the hydrogen form of an ion exchanger. In these processes through neutralization there is formed a sol which is azeotropically dewatered and then dried. Finally there has also been described the hydrophobization by esterification with alcohols at elevated temperatures. Also in these processes the excess alcohol must be recovered by distillation.
In another known process the pasty silica produced by wet precipitation followed by filtration or centrifuging is subsequently washed and treated with alkaline material. Thereby a pH of 8 to 12, preferably 9 to 10 is established. Then silicone oil or a silicone oil suspension is added and after this treatment the composition is dried, for example, at temperatures of 100.degree. to 120.degree. C. The addition of the silicone oil or the silicone oil emulsion takes place simultaneously with or after a preceding liquefaction of the filter cake or the centrifuge residue. This liquefaction takes place through the effect of suitable mechanical forces in the above-mentioned pH range. The desired pH value is attained by addition of an alkali solution or aqueous ammonia. The silicone oil treated silica obtained in this way according to an advantageous form of the invention before the drying can be freed of excess liquid by expressing.
The silica treated by the process of the invention can be used with advantage where it is important to have a small wetting angle of the silica against water; for example in cable mixtures, water resistant vulcanizates, as thickening agents for water resistant lubricants and non-foaming cosmetics.
All known coating processes have the object of covering the large active surface of the finely divided wet precipitated silica with as dense as possible layer of organic groups. Thereby the hydroxyl groups are bound or shielded by organic radicals so that they will no longer have effect as active fillers for elastomers and other polymers. Moreover, because of the easy breakdown of the thin organic layer these products lose their hydrophobic properties very easily. Apart from this the previously known hydrophobizing processes are very cumbersome and time consuming. This is especially true in those cases in which an organic solvent must be recovered.
To overcome the previously mentioned disadvantages according to a further known process a hydrophobic, coated, outstanding silica can be produced having a high resistance against saponifying agents by precipitating alkali silicate solutions with mineral acids and treating with organohalosilanes. Thus there is added to the acidic precipitated suspension recovered in known manner, at an elevated temperature of about 50.degree. to about 90.degree. C., an organohalosilane, the precipitated product filtered off, washed and dried and then subjected to a temperature treatment at about 300.degree. to about 400.degree. C. The product obtained has a loose structure and can be easily ground. The hydroxyl group found in the silica surface are further broken down by the temperature treatment whereby the hydrophobic properties are strengthened. Since the treatment is carried out in the precipitated suspension produced in known manner the use of special hydrophobizing apparatus is superfluous. Besides the suspension of precipitated silica can be washed quicker and more completely than the untreated suspension. It is possible to obtain a nearly electrolyte free product.
The process is suitably carried out by producing an acid precipitated suspension, for example, having a pH value of about 5 and preferably at a higher temperature of about 50.degree. to about 90.degree. C. the organohalosilane is added as slowly as possible using thorough stirring. Then it is recommended for completion of the reaction to stir further for some time. After the filtration the product is washed until neutral and the filter cake dried, for example in a shelf drier. The dried product is then admitted to the temperature treatment at about 300.degree. to about 400.degree. C. which can be done for example in a muffle furnace. Through this a breakdown of the hydroxyl groups takes place. Depending upon the desired degree of hydrophobization this can be continued for any desired time.
The hydrophobic properties can further be regulated by the amount of organohalosilane used. Generally it is sufficient to add amounts of above 10 to about 20 weight percent, based on the precipitated dry silica.
However, the last process has the disadvantage that the cost of silane is the greatest part of the cost of the crude material costs. Therefore a saving of silane would effect a considerable reduction in cost. There have already been tried experiments in which silane use is reduced by employing silicas of lower surface area. The effect was only slight if the saving does not take place in the composition in which the silica surface is reduced.
The hydrophobization degree of a silica can be ascertained very well by determination of the DBA number (dibutylamine number). The adsorption of dibutylamine is specific for acid groups, i.e., in the case of the silica the amine is only adsorbed by the silanol groups. If a silica is treated with silane with increasing addition of silane a reduction of the hydrophilic portion of the surface is observed and therewith the DBA-number. This is also established by determination of the methanol wettability, however, this method is first usable at a certain hydrophobicity. If there is pursued the change of DBA-number and therewith the dydrophobicity of a silica with increasing addition of silica, it is established that the change of DBA-number is always smaller with increasing silane addition. This means, if it is assumed, a fixed amount of silane covers a fixed surface that the silane yield is always smaller with increasing degree of hydrophobization. This is understandable since the silane concentration in the solution remains constant, the other reactant, however, the hydrophilic part of the silica, always decreases.